List all users in a multi tenant environment

Is there any way to list all users across all tenants? It looks like I can't just log in as super-admin and issue a simple call to a service. Do we somehow have to log in to each of the tenants and fetch all the users? Or am I missing something?

From what I can tell the PersonService only returns users in the current tenants you are logged on to. Are there any other services I can use to fetch all users across tenants?

The services are deliberately partitioned for a MT instance. As you suggest, if you wish to list all users across all tenants then you could write a custom web script that allows the super admin (an admin user that belongs to the default domain - eg. "admin") to loop through each tenant. You could also filter by tenants that are enabled /disabled, if appropriate.